The turbos would still be operating at the same pressure the difference is in the amount of air that they can put into the chamber when the valves open.
So even though the pressure is the same, due to the TD05 being larger it can move a greater volume at a higher speed, which gives you a bigger bang.
You can get smaller turbos to run higher pressures by modifying the actuator or fitting bleed off valves, but this in itself wont necessarily give more performance
What I would investigate - and will be doing for my own car is the standard size turbo but getting one with a roller bearing setup - this allows the turbo to spool up faster and gives increased component reliability.
Also it depends on what type of TD04 you have as there are several variants for the subaru range (TD04 - 13G, TD04 - 19T, TD04 - 17G) and others that will fit but wont necessarily be any good for the car - for instance the TD04 on a forrester is different to the TD04 on an impreza.
this link has a lot of info but.... it is comprehensive and illustrates the compressor maps - showing the optimum flow rate for size of turbo - this is the info that will help you ensure bolting on a different turbo will get gains
http://www.stealth316.com/2-3s-compflowmaps.htm